1. Technical Field
The present invention relates to a flow sensor, and in particular, to a reduction in errors of and an improvement in sensitivity of a flow sensor for a thermal mass flowmeter that is adapted to detect a mass flow rate based on a change in temperature distribution of a narrow tube through which a fluid is passed while the tube is being heated.
2. Related Art
Generally, heating of and measurement of temperature of a narrow tube of a flow sensor for a thermal mass flowmeter is performed by applying an electric current through temperature sensing resistors provided on the narrow tube.
A conventional flow sensor for a fluid flowmeter is disclosed, for example, in U.S. Pat. No. 3,938,384, wherein a sensor is provided for detecting a flow rate of a fluid flowing through a heated narrow tube, a thermally insulating material being used in the sensor so as to prevent convection at an outer space of the narrow tube. When the sensor is positioned at a different angle, the outer convection generates a temperature gradient on the heated narrow tube, which causes a flow sensing error. This error is called an attitude error. As the thermally insulating material, a material such as glass wool is generally now used. However, such a material has some disadvantages; for example, an amount of the material to be used cannot be easily controlled when manufacturing and properties of the material change under the influence of humidity (see U.S. Pat. No. 3,938,384).
U.S. Pat. No. 4,517,838 also discloses a structure having: a spacer between a pair of wound coils functioning as temperature sensing resistors; and a narrow groove surrounding an outer space of a narrow tube. This structure is designed to reduce air convection generated outside the narrow tube when a sensor is positioned at a different angle and the attitude errors associated with the convection. However, the spacer is located at a position on the narrow tube where the temperature reaches the highest. Accordingly, there is a disadvantage that heat loss due to heat dissipation from this position arises. Another disadvantage is that the narrow groove absorbs heat, causing a reduction in sensitivity (see U.S. Pat. No. 4,517,838).
In U.S. Pat. No. 4,815,280, multilayered coils are used as temperature sensing resistors. Such a multilayered structure is designed to reduce a coil width and a response time. However, this structure is also subject to a problem in that it is difficult to obtain a sensor having high sensitivity, because of heat transmission in an outer circumferential portion of the coil. Further, a number of problems exist in manufacturing, in particular, it is extremely difficult to produce a very fine, uniform multilayered coil (see U.S. Pat. No. 4,815,280).